Recently, Beyond 3G has been developed as the next-generation radio network which can establish seamless and safety connection between a plurality of radio communication systems including third generation mobile communications (3G), wireless LANs and fourth generation mobile communications (4G). As the uplink transmission scheme for the Beyond 3G, use of a single carrier transmission scheme is considered (e.g., see literature ‘3GPP, “TR25.814 vl.2.2” March 2006’(which will be referred to hereinbelow as literature 1)).
FIG. 1 is a diagram showing a configuration of a transmitter based on a single carrier transmission scheme described in literature 1.
The transmitter shown in FIG. 1 includes data transmitter 1101, pilot transmitter 1102, MUX portion 1103 for multiplexing these outputs.
Further, data transmitter 1101 includes DFT (Discrete Fourier Transformation) portion 1111, subcarrier mapping portion 1112, IFFT (Inverse Fast Fourier Transformation) portion 1113 and cyclic prefix adder 1114.
Data transmitter 1101 shown in FIG. 1 operates as follows:
First,
data made up ofNTx_d  [Math 1]symbols,
is transformed into a frequency-domain signal by applying DFT atNTx_d  [Math 2]points to the data at DFT portion 1111. Then, in subcarrier mapping portion 1112 the frequency-domain signal is mapped onto sub-carriers (by inserting ‘0’ into unused subcarriers
to form data of subcarriers amounting toNFFT_d).  [Math 3]
Then, the frequency-domain signal after the subcarrier mapping is
transformed into time-domain signal by applying IFFT atNFFT_d  [Math 4]points to the data at IFFT 1113. Finally, in cyclic prefix adder 1114, the data is added with a cyclic prefix to be transmitted.
FIG. 2 is a diagram showing how a cyclic prefix is added in cyclic prefix adder 1114 shown in FIG. 1.
Cyclic prefix addition at cyclic prefix adder 1114 shown in FIG. 1 is to copy the rear part of the block to the front end of the block as shown in FIG. 2.
It should be noted that a cyclic prefix is inserted in order to efficiently perform frequency-domain equalization on the receiver side. The cyclic prefix length is preferably set so as not to exceed the maximum delay time of the delay path in the channel.
Next, a configuration of a typical receiver corresponding to the transmitter shown in FIG. 1 will be described.
FIG. 3 is a diagram showing a configuration of a typical receiver corresponding to the transmitter shown in FIG. 1.
The receiver shown in FIG. 3 includes: DeMUX portion 1301 for separating the signal transmitted from the transmitter shown in FIG. 1 into a data signal and a pilot signal; data receiver 1302; and pilot receiver 1303.
Further, data receiver 1302 is comprised of cyclic prefix remover 1311, FFT (Fast Fourier Transformation) portion 1312, subcarrier demapping portion 1313, frequency equalizer 1314, IDFT (Inverse Discrete Fourier Transformation) portion 1315 and data demodulator 1316.
Data receiver 1302 shown in FIG. 3 operates as follows:
First, at cyclic prefix remover 1311, the cyclic prefix is removed from the received signal. Then,
the data is transformed into a frequency signal by applying FFT atNFFT_d  [Math5]points in FFT portion 1312. Then, the signal is demapped into subcarriers used by each user at subcarrier demapping portion 1313. After demapping, the signal is subjected to frequency-domain equalization at frequency equalizer 1314, based on the channel estimate obtained by channel estimator 1324 (described later) of pilot signal receiver 1303. Then,
the signal is transformed into time-domain signal by applying IDFT atNTx_d  [Math6]points in IDFT portion 1315, and then the received data is demodulated at data demodulator 1316.
Next, the uplink pilot signal and a user multiplexing method will be described.
Recently, as a pilot signal sequence, CAZAC (Constant Amplitude Zero Auto-Correlation) sequences have drawn attention. For example, as one of CAZAC sequences, the Zadoff-Chu sequence expressed by formula 1 can be considered (e.g., see a literature ‘B. M. Povic, “Generalized Chirp—Like Polyphase Sequences with Optimum Correlation Properties,” IEEE Transactions on Information Theory, Vol. 38, No. 4, pp 1406-1409, July 1992).
                                              ⁢                  [                      Math            ⁢                                                  ⁢            7                    ]                                                                                          c            k                    ⁡                      (            n            )                          =                  {                                                                      exp                  [                                                                                    j2π                        ⁢                                                                                                  ⁢                        k                                            L                                        ⁢                                          (                                                                                                    n                            2                                                    2                                                +                        n                                            )                                                        ]                                                                                                                                                when                        ⁢                                                                                                  ⁢                        the                        ⁢                                                                                                  ⁢                        sequence                                                                                                                                                length                        ⁢                                                                                                  ⁢                        L                        ⁢                                                                                                  ⁢                        is                        ⁢                                                                                                  ⁢                        even                                                                                                                                                                  exp                  [                                                                                    j2π                        ⁢                                                                                                  ⁢                        k                                            L                                        ⁢                                          (                                                                        n                          ⁢                                                                                    n                              +                              1                                                        2                                                                          +                        n                                            )                                                        ]                                                                                                                                                when                        ⁢                                                                                                  ⁢                        the                        ⁢                                                                                                  ⁢                        sequence                                                                                                                                                length                        ⁢                                                                                                  ⁢                        L                        ⁢                                                                                                  ⁢                        is                        ⁢                                                                                                  ⁢                        odd                                                                                                                                                    (                  Formula          ⁢                                          ⁢          1                )            
A CAZAC sequence is a sequence that has a constant amplitude in both time and frequency domains and produces self-correlation values of ‘0’ other than when the phase difference is ‘0’. Since the sequence is constant in amplitude in the time domain, it is possible to suppress the PAPR (Peak to Average Power Ratio), and since the sequence is also constant in amplitude in the frequency domain, the sequence is suitable for channel estimation in the frequency domain. Further, since the sequence has an advantage of being suitable for timing detection of the received signal because it has perfect self-correlation characteristics, the sequence has drawn attention as a pilot sequence that is suitable for single carrier transmission, which is the access scheme for uplink of Beyond 3G.
As the user multiplexing method when CAZAC sequences are used as the pilot signal sequences for uplink, Code Division Multiplexing (CDM) has been proposed (e.g., see literature ‘3GPP, R1-051062, Texas Instruments “On Uplink Pilot in EUTRA SC-OFDMA”, October 2005’).
In CDM of the pilot signals, all the users use CAZAC sequences of an identical sequence length added with a cyclic shift unique to each user. If the cyclic shift time is taken to be equal to or longer than the expected maximum delay, the pilot signals of all the users under the multipath environment can be made orthogonal to one another. This is feasible based on the property that the self-correlation value of a CAZAC sequence constantly becomes ‘0’ other than when the phase difference is ‘0’.
The transmitter and receiver of a pilot signal when the pilot signal undergoes CDM will be described with reference to FIGS. 1 and 3. Since the basic configuration and operation of pilot transmitter 1102 is the same as data transmitter 1101, the points different from data transmitter 1101 will be described.
To begin with, the numbers of points for DFT portion 1121 and for IFFT portion 1123 areNTx_p,NFFT_p  [Math8](in literature 1, these are defined asNTx_p=NTx_d/2,NFFT_p=NFFT_d/2).  [Math 9]
When user multiplexing of pilot signals is performed by CDM, in order to separate the users from each other at the receiver, cyclic shift portion 1124 performs a cyclic shift unique to the user. A cyclic shift is a shift whereby the pilot signal sequence is handled like a ring, and the pilot signal sequence is reentered from the last end into the front end as shown in FIG. 2. The amount of the cyclic shift for each user is preferably equal to or greater than the maximum delay of the delay path, or the cyclic prefix length. Finally, the cyclic prefix is added at cyclic prefix adder 1125, and the generated data signal and the pilot signal are time-multiplexed through MUX portion 1103 to be transmitted.
Next, pilot receiver 1303 will be described.
In pilot receiver 1303, the data signal and the pilot signal are separated from each other by DeMUX portion 1301, then the cyclic prefix is removed by cyclic prefix remover 1321. Then, the pilot signal
is subjected to FFT atNFFT_p  [Math 10]points by FFT portion 1322, so as to be transformed into the pilot signal in the frequency domain. Then, subcarrier demapping is performed at subcarrier demapping portion 1323, thereafter, channel estimation is performed by channel estimator 1324. The channel estimate for each user is output to frequency equalizer 1314 of data receiver 1302.
When CAZAC sequences are used in a cellar system, the cross-correlation characteristic is also important. In view of inter-cell interference suppression, it is preferred that a group of sequences that yield small cross-correlation values are allotted as the pilot signal sequences for neighboring cells. The cross-correlation characteristics of a Zadoff-Chu sequence greatly depend on the individual sequence. For example, when the sequence length L of a Zadoff-Chu sequence includes a prime or a large prime, it presents excellent cross-correlation characteristics (a low cross-correlation value). On the other hand, when it is a composite number composed of small primes only, the cross-correlation greatly degrades (the cross-correlation value contains a large value). Specifically, the sequence length L of Zadoff-Chu sequences is a prime, the cross-correlation value between arbitrary Zadoff-Chu sequences is considered to be kept constant at(1/L)1/2  [Math 11](see non-patent document 3, for example).
In Beyond 3G, it is assumed that the transmission bandwidths of data signals and control signals differ from one user to another. Accordingly, the pilot signal used for demodulation of the data signal and control signal differs in transmission bandwidth for every user, hence it is necessary to multiplex the plot signals of users different in transmission bandwidth.
FIG. 4 is a diagram showing one configurational example a conventional mobile radio system.
In the mobile radio system shown in FIG. 4, constituted of BS1001 as a base station and CL1000 as a service area formed by BS1001, a plurality of mobile stations MS1002-1005 for performing communications with the BS1001 are provided.
FIG. 5 is a diagram showing frequency blocks used by the users in the mobile radio system shown in FIG. 4 and one example of pilot signal sequences used for the individual users.
As shown in FIG. 5, when the data signal or control signal is transmitted with a single carrier using a frequency block having continuous frequencies, the pilot signal is also transmitted with a signal carrier using the same frequency block as that of the data signal or control signal.
In a case where CDM is used to multiplex the pilot signal, when, in FIG. 5 for example, the bandwidths of the signals transmitted by MS1002-1005 are 3W, W, W and 2W (W is a predetermined bandwidth), CAZAC sequences having a sequence length of 3L, L, L and 2L corresponding to respective bandwidths will be used as the pilot signal sequences.
In this case, there is the problem that the pilot signals for the users who perform pilot signals using different frequency blocks of continuous frequencies will not become orthogonal. The reason is that the sequence lengths of the pilot signals are not the same between the users who use different frequency blocks.
FIG. 6 is a diagram showing another configurational example of a conventional mobile radio system.
In the mobile radio system shown in FIG. 6, constituted of BS1001 and 1301 as base stations and CL1000 and CL1300 as service areas formed by BS1001 and BS1301, a plurality of mobile stations MS1002-1005 and MS1302-1305 for performing communications with BS1001 and BS1301 respectively are provided.
FIG. 7 is a diagram showing frequency blocks used by the users in CL1300 of the mobile radio system shown in FIG. 6 and one example of pilot signal sequences used by the individual users. Here, the frequency blocks used by the users in CL1000 and the pilot signal sequences used by the individual users are assumed to be the same as those shown in FIG. 5.
Remarking the frequency blocks used in adjacent cells, for example, MS1003, or MS1004, and MS1303 have the same bandwidth, so that it is possible to suppress inter-cell interference using difference CAZAC sequences. In contrast, for example, MS1002 and MS1302, or MS1002 and MS1304 use frequency blocks different in bandwidth, hence it is impossible to suppress inter-cell interference. In other words, when CAZAC sequences used as pilot signals are different in sequence length, there is the problem that inter-cell interference cannot be suppressed. The reason is that the mutual correction characteristics between CAZAC sequences different in sequence length degrade.